The technique is this:
This mod does not use a modified form, only VBA. However, we're going to modify the form and then delete the modification.
So, open the form in Modifier.
Drag the Display Existing Record field from below onto the form, set it's Visible property to TRUE
Go into Modified Forms and Reports and set security to the modified form.
Open the form, add the new field to VBA. If you have existing VBA, you'll have to set it to emModifiedOnly
Close the form.
Go to Tools > Customize > Modifier
Delete the modified form
Go into security and set security back to the original form
Go into the VBA editor and add a Changed event for Display Existing Record
When this event fires, the form is completely populated... now we can set our default.